It was recently discovered that neurons can release endocannabinoids (eCBs) from their cell bodies and dendrites to activate presynaptic type 1 cannabinoid receptors (CB1Rs). Despite intensive study many fundamental questions about eCB signaling remain unanswered. It has been shown that eCBs can transiently decrease the probability of neurotransmitter release, but what is the physiological role of such short term plasticity? Does eCB release allow cells to globally regulate all of their synapses under physiological conditions? Does eCB signaling provide a general mechanism for short-term associative synaptic plasticity? Is eCB release specialized in different cell types to regulate the timing and frequency dependence of associative plasticity? Do eCBs allow target-dependent regulation of synapses? How is eCB signaling modulated? Previous studies have also shown that presynaptic CB1Rs control the induction of a postsynaptic form of long-term depression (LTD) in the cerebellum that plays an important role in motor learning. How do CB1Rs control this form of LTD? Do the properties of eCB release lead to a timing dependence for the induction of LTD that is suited to motor learning? eCBs can also regulate the excitability of some types of cells, although this aspect of eCB signaling is much less studied than synaptic regulation. Do eCBs regulate neuronal excitability for cells throughout the brain or is this a rare form of signaling? Why do eCBs allow some cells to regulate their own excitability whereas others are only influenced by eCBs released from neighboring cells? Why do eCBs lead to sustained changes in excitability in some cell types and only transiently affect others? These questions will be addressed by studying eCB signaling in brain slices from rats and mice. Studies will use whole-cell voltage clamp and current-clamp recordings to evoke eCB release, quantify changes in synaptic strength, monitor firing properties and measure effects on membrane potential. Postsynaptic calcium, which is a vital regulator of eCB release, will be measured and manipulated. Quantification of presynaptic calcium entry, which is regulated by CB1R activation, will provide a measure of eCB-mediated presynpatic modulation and will allow us to study individual presynaptic cells. Virtually all of the techniques required in this study are routinely used in the laboratory, making it likely that the proposed experiments will be completed in the allocated time. These studies will lead to a deeper understanding of the role of eCB signalling in the cerebellum, motor control and motor learning. They will also provide general insight into eCB signaling that will aid in the understanding of pain, epilepsy, appetite control, depression and Parkinson's disease.